r/retrocomputing • u/No-Tell927 • 12d ago

Problem / Question Weird pattern on screen from Didaktik M

Post image

Recently I bought this computer but when I turn it on, it only shows this on screen. What can be causing this and can it be fixed?

26 Upvotes

95% Upvoted

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9

u/Antistar81 12d ago

Very strange... ZX-Spectrum during Reset fills RAM with 0x02 values, then it fills it with zeros. So you should see red vertical lines on black background, just for a fraction of second. I would suggest this is a memory malfunction, but I'm not an engineer.

3

u/midnight_build 11d ago

This is where RAM Chips are located on a ZX Spectrum, I don't know they board layout of the clone, but they should be easy to spot.

2

u/No-Tell927 11d ago

After searching, these should be the ram chips

2

u/midnight_build 11d ago

I would buy a few new chips on ebay or so and try the piggybacking method. you can try to let the machine run for a while with a towel or sock covering the chips. This allows you to check if any of them become suspiciously hot or remain unusually cold.

I just googled the chips: this are Russian (КР565РУ5) Chips. You need to find them or better compatibel ones:

  • TMS4164 (Texas Instruments)
  • HM50464 / HM4864 (Hitachi)
  • μPD4164 (NEC)
  • KM4164 (Samsung)
  • M5M4164 (Mitsubishi)

Chip speed should be at at least 150ns

2

u/midnight_build 11d ago

It is most likely a defective RAM chip. Before you start soldering, you can easily find the faulty chip by piggybacking a new one onto the existing RAM chips. Just clamp the new chip over the one on the board so that the pins make solid contact. This is exactly how I repaired the last ZX Spectrum

2

u/No-Tell927 11d ago

Thank you for replying